1. Field of the Invention
The present invention relates to an optical system and apparatus for laser heat treatment, and to a method for producing semiconductor devices by laser crystallization by heat-treating a thin silicon thin film formed on a substrate to convert an amorphous or polycrystalline silicon thin film into coarse-grained silicon.
2. Prior Art
The pixel section of a liquid crystal display panel is constituted from thin film switching transistors formed from an amorphous or polycrystalline silicon thin film on a substrate of glass or synthesized quartz. A driver circuit for driving the pixel transistors has been arranged mostly outside the panel, but if such a driver circuit were formed together in the panel, tremendous advantages would be achieved in terms of reliability of the liquid crystal panel with lowered cost of production.
At the present, however, since the active layer of the transistors has been made of a silicon thin film of low crystallinity, the thin film transistor has poor performance exemplified by low mobility, thus making it difficult to incorporate an integrated circuit having the required high-speed operation and high performance to the display panel.
It has been found that crystallinity of thin film silicon and carrier mobility in the thin film transistors formed therefrom are related to each other, as described below. Thin film silicon for the pixel transistors has been obtained by laser heating of amorphous silicon for crystallization, which is generally polycrystalline having a great number of lattice defects concentrated in the grain boundaries of the crystallized silicon thin film which significantly impede carrier mobility in the active layer of thin film transistors. In order to improve mobility in the thin film transistors, such measures are taken as reducing the number of times the carriers cross the grain boundaries in the process of migration in the active layer, and decreasing the concentration of lattice defects.
Techniques to improve crystallinity of thin film silicon include heat treating with laser beam for providing higher mobility for thin film transistors. A purpose of the heat treating is to enlarge crystal grains and lessen lattice defects in the grain boundaries of the silicon thin films.
Attempts of heat treatment with laser have been reported by B. Rezeck in Jpn. Journal Appl. Phys. vol. 38 (1999) pp. L1083-L1084 2nd part No.10A, and by J. Carvalho, et al. in Mat. Res. Soc. Symp. Proc., Vol.358, 1995, pp915-920. Laser light used in the works reported in these references is the second harmonic (wavelength 532 nm of light) generated by an Nd:YAG laser. FIG. 12 shows such an example of laser heat treatment apparatus that includes an optical system for laser heat treatment using the second harmonic of an Nd:YAG laser. An oscillator 1 in the apparatus uses the second harmonic (wavelength 532 nm) generated by an Nd:YAG laser, which is a representative pulsed laser source of visible light used for the heat treatment application. A laser beam 2 from the laser apparatus is focused by a condenser lens 4 to irradiate and heat an amorphous or polycrystalline silicon thin film 5 which has been previously deposited on a substrate 7 via a base layer 6. The amorphous silicon layer 5 in the irradiated area is fused by the heat generated by the pulse laser beam 2, and is then cooled to crystallize into coarse-grained silicon layer on the substrate.
According to the reports quoted above, the conventional laser beam has a profile characterized by a rotationally symmetrical Gaussian distribution at the point of irradiation, thus causing crystal grains to grow in radial directions in a rotationally symmetrical pattern, as shown in FIG. 13, in the crystallization process of the molten silicon. Consequently, since the polycrystalline silicon thin film has very poor uniformity within the plane after heat treatment with a laser beam, there has been reported no attempt of producing thin film transistors with this technique.
Meanwhile, an excimer laser having shorter wavelength has been used in heat treatment utilizing a linear beam profile. This is based on a completely different concept from that of heat treatment with laser light of a wavelength not shorter than 330 nm. Since the heat treatment with laser light of a wavelength not shorter than 330 nm causes crystalline growth of the molten silicon within the plane, namely in the horizontal directions as described above, it is employed for forming large crystal grains. On the other hand, since heat treatment with an excimer laser beam causes crystal growth in the direction of film thickness (vertical direction), it is employed merely for the purpose of improving the uniformity of the film quality within plane after heat treatment with a laser beam and improving the productivity, not for the purpose of growing large grains.
An optical system for forming a linear beam profile from a laser beam generated by an excimer laser is disclosed in Japanese Patent Publication Nos. 11-16851 and 10-33307. The laser beam emitted by the excimer laser oscillator is, after passing a cylindrical lens array arranged in two directions intersecting at right angles with each other in a plane perpendicular to the optical is of the beam, concentrated by a focusing lens and is processed by a beam homogenizer that equalizes the intensity distribution in two directions, resulting in converging widths that are different in the two directions.
In a heat treatment using a laser beam that has a rectangular cross section, the light intensity distribution profile must be optimized in order to produce thin film transistors having excellent characteristics. This is because the intensity distribution profile in the beam width direction has especially great effect on the growing process of crystals, and the distribution in the longitudinal direction governs the region where the crystal grow. However, the conventional optical systems used for forming linear beams do not allow selection of a proper profile in the direction of width. Also because the beam is homogenized in two perpendicular directions, it has not been possible to converge the linear beam to an extremely small width.
The present invention has been made to solve the problems described above. An object of the present invention is to provide an optical system that controls a light intensity distribution of laser beam into a optimized profile for producing excellent crystallinity in a thin film with coarse crystal grains and reduced lattice defects which is required for producing high performance of thin film transistors.
Another object of the present invention is to provide an optical system for achieving a laser beam shape of extremely narrow rectangular shape suitable to relatively scan the beam across a thin film on the substrate and a very steep light intensity distribution in the direction of scanning the film surface.
Still another object of the present invention is to provide a laser heat-treating apparatus for forming excellent crystallinity of a thin film silicon required for producing high performance for thin film silicon transistors.
A further object of the present invention is to provide a method of producing a thin film semiconductor of excellent crystallinity required for producing higher performance thin film transistors.
An optical system for laser heat treatment according to the present invention includes an intensity distribution forming means for controlling the light intensity distribution of a laser beam emitted by a laser oscillator for the purpose of heat-treating a film material formed on a substrate by irradiating laser light thereon, and a beam shape forming means for forming a rectangular beam shape on a film on the substrate. The intensity distribution forming means makes uniform distribution of the light intensity of the laser beam emitted from the laser oscillator in one direction within the cross section of the beam, while maintaining the primary intensity distribution of the laser beam as emitted by the laser oscillator to the other direction perpendicular to the former direction, and then the beam shape forming means changes the beam having such intensity distribution from the intensity distribution rig forming means into a beam rectangular in cross section and projects this laser beam onto the film material to be heated. The beam shape forming means may enlarge and/or reduces the beam profile in the one direction and/or in the other direction before passing through the beam shape forming means to achieve uniform heating of the film material.
The optical system of the invention enables possible control of the temperature distribution on the film and achieves a laser beam spot profile for uniform heating.
An intensity distribution forming means in the present invention may cause a part of the laser beam reflected a plurality of times in one direction in the cross section of the laser beam, and combines the reflected part and straight passing part of the laser beam, thereby to form a uniform light intensity distribution of the beam. Such an intensity distribution forming means preferably may includes a pair of reflecting planes opposed to each other with a distance therebetween.
The intensity distribution forming means may employ a waveguide structure for limiting a spread of a primary laser beam having a Gaussian energy distribution along only one direction in the cross section of the laser beam, thereby to form uniform intensity distribution to said direction. The intensity distribution forming means may also use a waveguide having a pair of reflecting planes which are opposed to each other and parallel or tapered to the direction.
Another intensity distribution forming means may employ a pair of separated cylindrical lens with a distance therebetween which are separated in one direction in the cross section of the laser beam, to focus parts of the beam to the one direction and straiten in the other perpendicular direction, thereby to form a uniform intensity distribution to the one direction while maintaining the Gaussian distribution in the other direction.
On the other hand, a beam shape forming means in the invention functions to project the beam having uniform intensity distribution in the one direction from the intensity distribution forming means onto a film to be heated on the substrate. Also, in projecting the beam, the beam shape forming means may be arranged in such a configuration that a uniform intensity distribution obtained from the intensity distribution forming means in the one direction is transferred with a suitable rectangular shape to the film, thereby to determine a shape of the longitudinal direction of the rectangular beam projected on the film. As a example of a simple beam shape forming means, a transfer lens such as a spherical lens may be used for transferring.
Further, the beam shape forming means may also focus the laser beam emitted by the from the intensity distribution forming means onto the film only in at least one direction in the cross section of the beam, thereby determining the profile of the direction of the shorter side of the rectangular beam spot on the film. The beam shape forming means may be a condenser lens that is used to focus the light onto the film. A cylindrical lens may be used for the condenser lens. In this lens configuration, a steep light intensity distribution may be determined in the direction of the shorter side of the rectangular beam shape on the film on the substrate.
The beam shape forming means may also include a combination of plurality of cylindrical lenses and/or spherical lenses. These configurations can form a uniform intensity distribution in the direction of longer side of the rectangular beam shape on the film on the substrate as well as a steep light intensity distribution may be formed in the direction of the shorter side, while irradiating laser beam of any desired shape. All or some of the plurality of cylindrical lenses or spherical lenses may be aspherical lenses. The light intensity distribution in the direction of the shorter side of the rectangular beam shape on the film can be made steep up to the limit permitted by directivity nature of the laser beam.
The optical system for laser heat treatment of the present invention may include a knife edge fitted in proximity to the film in parallel to the direction of the longer side of the rectangular beam shape on the film. The knife edge is capable of defining the beam shape and making the light intensity distribution steeper.
In the invention, the optical system may preferably use a pulsed laser for heating a thin film applied on a substrate. Particularly, the optical system for laser heat treatment may be provided with a pulse width extension means which separates a primary pulsed laser beam emitted form a oscillator into at least two different optical paths having different path lengths, and thereafter overlap the partial beams in a single path to extend a range of laser plus time. Such a pulse width extension means may be arranged anywhere between the laser oscillator and a beam shape forming means, or, in some case, just in front of the substrate. Since pulse width of the laser can readily be established, duration of the crystal growth can be controlled according to different thickness of thin films.
The invention includes an apparatus for laser crystallization comprising the optical system for laser heat treatment described above and a stage that mounts a substrate on which the thin film have been applied. In particular, the apparatus of the present invention can be applied to production of semiconductor devices wherein the optical system scans the laser beam of rectangular cross section irradiated on the surface of a semiconductor film such as thin film silicon, applied on a substrate, continuously heating and cooling the surface to form coarse crystal grains in the semiconductor film during melting and crystallizing process.
The semiconductor film may be prepared of amorphous or polycrystalline silicon thin film deposited on the substrate. The coarse-grained silicon thin films fabricated by this method can be widely applied for producing thin film transistors for visual signal processing use.
A laser oscillator in the laser heat treatment apparatus of the present invention may generate a laser beam having wavelength in a range from 330 nm to 800 nm; this wavelength allows thin film silicon to be heated uniformly in the direction of thickness of the silicon thin film.